A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - Valley slope sections in Jurassic strata near Bath, Somerset

The valleys of the river Avon and its tributaries in the Bath area are characterized by steep slopes, up to 15°, and high relief, typically up to 160 m. The considerable down­ cutting of the river during the Devensian period, amounting to about 27 m, caused several large landslips, but cambering and valley-bulging appear to have been com­ pleted before the Last (Ipswichian) Interglacial, probably in the Wolstonian glacial period. Cambering is associated with disturbance of the strata to depths of 30-40 m and is present in the more gentle slopes which have been left essentially undisturbed by subsequent erosion. Conversely, the steepest and most actively eroding slopes are not cambered, the disturbed material probably having been removed by landslides and mudflows. The slopes are mostly blanketed by colluvium or Head, ranging from 1 to 5 m in thickness. In general the Head is only marginally stable under present climatic condi­ tions and the angle of limiting equilibrium seems to be related to the thickness of colluvium, in response to the variations of pore pressure and shear strength with depth. The observed lower bound of 9° in the Fuller’s Earth clay slopes, with 5 m of Head, may well be at or close to the angle of ultimate stability.